1. Field of the Invention
Present invention relates to a wireless communication system, especially to an apparatus and method for allocating uplink ACK/NACK channels for downlink data transmission in a wireless communication system.
2. Description of the Related Art
3GPP (the 3rd Generation Partnership Project) Standardization organization is taking long-term evolution (LTE) over regulations for existing systems. Its downlink transmission technique is based on Orthogonal Frequency Division Multiplexing (OFDM), and uplink transmission technique is based on Single is Carrier Frequency Division Multiple Addressing (SCFDMA). There are two types of frame structures in LTE system, in which type 1 of frame structure applies FDD and type 2 applies TDD.
FIG. 1 shows a frame structure in LTE FDD system in which a time duration of radio frame is 307200×Ts=10 ms and each frame is divided into 20 time slots 15360Ts=0.5 ms long, the slots have indexes ranging from 0 to 19. Each time slot includes a plurality of OFDM symbols whose CP has two types, i.e., a normal CP and an extended CR Time slots using normal CP include 7 OFDM symbols while the time slots using extended CP have 6 OFDM symbols. Each sub-frame consists of two successive time slots, i.e., a kth sub-frame includes a slot 2kth and a slot (2k+1)th.
FIG. 2 illustrates a frame structure in LTE TDD system. Radio frame whose length is 307200×Ts=10 ms is divided into two equal half-frames 153600×Ts=5 ms long. Each half-frame includes 8 slots with 15360Ts=0.5 ms long and 3 special domains, i.e., a downlink pilot time slot (DwPTS), a guard period (GP) and an uplink pilot time slot (UpPTS), a total length of the three domains is 30720Ts=1 ms. Each time slot includes a plurality of OFDM symbols whose CP has two types, i.e., a normal CP and an extended CP. Time slots using normal CP include 7 OFDM symbols while the time slots using extended CP have 6 OFDM symbols. Each sub-frame consists of two successive time slots, i.e., the kth sub-frame includes the 2kth and (2k+1)th time slots. Sub-frame 1 and 2 include the 3 special domains mentioned. According to the discussion result, sub-frame 0 and 5 and DwPTS are constantly assigned for downlink transmission. If a conversion period is 5 ms, UpPTS, sub-frame 2 and 7 are constantly assigned for uplink transmission. If the conversion period is 10 ms, UpPTS and sub-frame 2 are constantly assigned for the uplink transmission.
According to the discussion result up to now, first N OFDM symbols in each downlink sub-frame are adopted to transmit downlink control channels. Here, for FDD, n is less than or equal to 3; for TDD, consideration shall be given into the situation that different requirements for downlink and uplink allocated proportions may require n shall be equal to or greater than 4. Physical control format indicator channel (PCFICH) is adopted to transmit the values of above said n, so that physical downlink control channel (PDCCH) is transmitted in the first n OFDM symbols. Here, PCFICH is transmitted in the first OFDM symbol of each frame and PDCCH is obtained by combining one or more control channel elements (CCE). Each CCE contains sub-carriers in fixed number. According to the discussion result up to now, the number of CCE contained in PDCCH may be 1, 2, 4 and 8. Moreover, whether it supports 3 CCEs to form a PDCCH or not is still under discussion.
According to the discussion result on LTE, physical time frequency resource is divided into a plurality of resource blocks (RBs) which are a minimum grain sizes for resource distribution. Each resource block includes M successive sub-carriers in frequency domain, and N successive symbols in the time, which is OFDM symbols in corresponding downlinks and SCFDMA symbols in corresponding uplinks. According to the discussion result on LTE up to now, M is 12, and N is subject to the number of OFDM or SCFDMA symbols in a sub-frame.
According to the discussion result on corresponding uplink control channel in LTE, the uplink control channel includes an ACK/NACK and a channel quality indicator (CQI), etc. When the uplink data transmission does not exit, the uplink control channel is allocated in preserved frequency domains as shown in FIG. 3 which are distributed at both ends of frequency band in the system. Meanwhile, to obtain frequency diversity effect, in a sub-frame, the uplink control channel occupies a RB (301) at the upper end of frequency band in the first time slot and a RB (302) at the lower end of frequency band in the second time slot, or a RB (303) at the lower end of frequency band in the first time slot and a RB (304) at the upper end of frequency band in the second time slot. Therefore, time frequency resource occupied by each control channel is allocated at both ends of frequency band in the system and its number is equal to that of time frequency resource for a RB. According to the discussion result up to now, for the frame structure adopting normal CP, the number of ACK/NACK channels multiplexed in each RB may be 36, 18 or 12; for the frame structure using extended. CP, the number of ACK/NACK channels multiplexed in each RB may be 12 or 8. In addition, when uplink data transmission exits, uplink control signaling is transmitted in uplink data channel resource allocated by node B.
According to the discussion result on downlink data transmission based on HARQ in LTE, for non-persistent scheduling, namely dynamic scheduling, indexes of ACK/NACK channels are bound impliedly with the minimum indexes of CCEs which form a PDCCH. According to the discussion result up to now, n, the number of OFDM adopted to transmit downlink control channels in each sub-frame is configured dynamically through PCFICH, so that the number of CCEs available actually in each downlink sub-frame is also configured dynamically through PCFICH; meanwhile, only one part of these available CCEs are adopted to schedule downlink data transmission dynamically. Since only CCEs adopted to schedule downlink data transmission dynamically requires to be bound with uplink ACK/NACK channels actually and the number of ACK/NACK required actually for downlink transmission of each sub-frame changes dynamically. However, the number of ACK/NACK in uplink direction is configured semi-statically, so that in general, only a part of them are occupied. When all ACK/NACK channels in one or more RBs adopted to transmit ACK/NACK information, configured semi-statically are not occupied, these RBs can be allocated and adopted to schedule uplink data dynamically in order to make full use of resources in the system.
FIG. 4 shows a schematic diagram for scheduling uplink data dynamically in a RB configured semi-statically and adopted to transmit ACK/NACK. Here suppose that ACK/NACK channel indexes bound with CCE are regarded as indexes of this CCE and the number of multiplexed ACK/NACK channels in each RB is 8. As shown in FIG. 4, a part of CCEs in current sub-frame are adopted to schedule downlink data transmission including 3 PDCCHs which consist of 4, 2 and 2 CCEs respectively. Therefore, there are only 3 occupied actually ACK/NACK channels distributed in the first RB but all ACK/NACK channels in the second RB are available, so that the second RB is idle, which node B can schedule dynamically to transmit uplink data transmission.
FIG. 4 shows a method for binding CCE and ACK/NACK, in which indexes of ACK/NACK channels bound with a CCE are equal to indexes of this CCE. However, under some circumstances, this method fails to reduce effectively the number of RBs in which ACK/NACK occupies. As shown in FIG. 5, suppose that the number of multiplexed ACK/NACK channels in each RB is 8 and 8 CCEs form a PDCCHs, two of which are sent in downlink sub-frame. As shown in FIG. 5, there are 2 ACK/NACK channels occupied actually but both of which belong to different RBs respectively. In order to ensure ACK/NACK channel performance, these two RBs can not be adopted to schedule dynamically uplink data transmission, which results in a failure for putting uplink resource into full use.
In LTE TDD system, configuring locations of switching points for downlink and uplink is able to adjust sub-frame proportion used for downlink and uplink transmissions. According to current results, for the 5 ms switch period, the possible proportion may be 1:3, 2:2 or 3:1; for a 10 ms switching period, the possible proportion may be 6:3, 7:2, 8:1 or 3:5. For the configuration that downlink sub-frame dominates, as downlink sub-frames are more than uplink ones, when ACK/NACK channels are bound for data transmission in each downlink sub-frame, it may be necessary that a plurality of downlink sub-frames shall be bound to ACK/NACK channels in the same uplink sub-frame. Suppose an uplink sub-frame requires to transmit ACK/NACK channels of K downlink sub-frames and let the number of CCEs in K downlink sub-frames be Nk, k=0,1, . . . K−1 respectively. Here, it is indicated dynamically from PCFICH that the number of OFDM symbols adopted to transmit downlink control channel in each sub-frame is n, and then the number of CCEs in each sub-frame is known as Nk. A binding method is: firstly, N1 CCEs in a first downlink sub-frame shall be bound with a first N1 ACK/NACK channels in uplink sub-frame; it is followed by that N2 CCEs in a second downlink sub-frame shall be bound with the next N2 ACK/NACK channels in uplink sub-frame; the rest may be deduced similarly.
FIG. 6 shows a schematic diagram of the above method for binding CCEs in a plurality of downlink sub-frames to ACK/NACK channels in one uplink sub-frame. Here suppose 3 ACK/NACK channels in a downlink sub-frame are transmitted in the same uplink sub-frame and the number of CCEs in each sub-frame obtained through PCFICH is 4, 8 and 4. In this way, 4 CCEs in the first downlink sub-frame are bound with ACK/NACK channels 0, 1, 2 and 3 in uplink sub-frame; 8 CCEs in the second downlink sub-frame are bound with ACK/NACK channels 4˜11 in uplink sub-frame; 4 CCEs in the second downlink sub-frame are bound with ACK/NACK channels 12˜15 in uplink sub-frame.
The binding method shown in FIG. 6 has a problem that its reliability depends on correct receiving of PCFICHs in a plurality of downlink sub-frames. Specifically, in order to receive correctly control channels and identify their used ACK/NACK channels, UE scheduled in a second downlink sub-frame will receive correctly PCFICHs in both the second and the first downlink sub-frames, since in the method for binding ACK/NACK shown in FIG. 6, indexes of ACK/NACK channels bound with CCEs in the second downlink sub-frame depends on the total number of CCEs in the first downlink sub-frame. Similarly, UE scheduled in a third downlink sub-frame shall receive correctly PCFICHs in both the third and the first two downlink sub-frames, since in the method for binding ACK/NACK shown in FIG. 6, indexes of ACK/NACK channels bound with CCEs in the third downlink sub-frame depends on the total number of CCEs in the first two downlink sub-frames. It will be seen that, in addition to the first downlink sub-frame, the reliability for CCE correctly binding with ACK/NACK channels in other downlink sub-frames declines as it depends on correct receiving of PCFICHs in a plurality of sub-frames.